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2 Mendel’s Laws: ChromosomesHomologous pairs of chromosomes: contain genes whose information is often non-identical =allelesDifferent alleles of the same gene segregate at meiosis IAlleles of different genes assort independently in gametesGenes on the same chromosome exhibit linkage: inherited together

4 Gene MappingGene mapping determines the order of genes and the relative distances between them in map units1 map unit=1 cM (centimorgan)Alleles of two different genes on the same chromosome are cisAlleles of two different genes on different homologues of the same chromosome are trans

9 Gene MappingGenes with recombination frequencies less than 50 percent are on the same chromosome (linked)Two genes that undergo independent assortment have recombination frequency greater than 50 percent and are located on nonhomologous chromosomes or far apart on the same chromosome (unlinked)

11 RecombinationRecombination between linked genes occurs at the same frequency whether alleles are in cis or trans configurationRecombination frequency is specific for a particular pair of genesRecombination frequency increases with increasing distances between genes

12 Genetic MappingMap distance between two genes = one half the average number of crossovers in that regionMap distance=recombination frequency over short distances because all crossovers result in recombinant gametesGenetic map = linkage map = chromosome map

13 Genetic Mapping Linkage group = all known genes on a chromosomePhysical distance does not always correlate with map distance; less recombination occurs in heterochromatin than euchromatinLocus=physical location of a gene on chromosome

14 Gene Mapping: Crossing OverCrossing-over between genes on homologous chromosomes changes the linkage arrangement of alleles on a single chromosomeTwo exchanges between the same chromatids result in a reciprocal exchange of the alleles in the region between the cross-over points

16 Gene Mapping: Crossing OverCross-overs which occur outside the region between two genes will not alter their arrangementDouble cross-overs restore the original allelic arrangementCross-overs involving three pairs of alleles specify gene order = linear sequence of genes

19 Gene MappingMapping function: the relation between genetic map distance and the frequency of recombinationChromosome interference: cross-overs in one region decrease the probability of second cross-overCoefficient of coincidence=observed number of double recombinants divided by the expected number

20 Gene Mapping: Human PedigreesMethods of recombinant DNA technology are used to map human chromosomes and locate genesGenes can then be cloned to determine structure and functionHuman pedigrees and DNA mapping are used to identify dominant and recessive disease genes

21 Gene Maps: Restriction EndonucleasesRestriction endonucleases are used to map genes as they produce a unique set of fragments for a geneEcoR1 cuts ds DNA at the sequence = 5’-GAATTC-3’ wherever it occursThere are >100 restriction endonucleases in use, and each recognizes a specific sequence of DNA bases

23 Gene Maps: Restriction EnzymesDifferences in DNA sequence generate different recognition sequences and DNA cleavage sites for specific restriction enzymesTwo different genes will produce different fragment patterns when cut with the same restriction enzyme due to differences in DNA sequence

28 Human Gene MappingHuman pedigrees can be analyzed for the inheritance pattern of different alleles of a gene based on differences in STRPs and SNPSRestriction enzyme cleavage of polymorphic alleles differing RFLP pattern produces different size fragments by gel electrophoresis

34 Neurospora: Meiotic SegregationProducts of meiotic segregation can be identified by tetrad analysisMeiosis I segregation in the absence of cross-overs produces 2 patterns for a pair of homologous chromo- somesMeiosis II segregation after a single cross-over produces four possible patterns of spores